JPS6194213A - Head wear detecting circuit - Google Patents

Abstract

PURPOSE:To avoid malfunctions and to ensure the assured detection for head wear by using a detecting means for the terminal voltage of a rotary magnetic head, a means for output of the reference voltage and a comparator means which compares the voltage given from said voltage detecting means with the voltage given from said reference voltage detecting means. CONSTITUTION:A transistor TR4 supplies a recording current to an audio head 7 serving as a rotary magnetic head and detects the produced AC voltage V through a detecting circuit 11 to obtain the DC detecting voltage VDET. The voltage V is reduced with reduction of the inductance; while the voltage VDET increases. A fixed level of current is secured at the emitter side of the TR4 and therefore the voltage has virtually no change at both ends of an emitter resistance 5. This terminal voltage is detected by a voltage detecting circuit 13 via an amplifier 12 to obtain the DC reference voltage VREF. An operational amplifier 15 compares the voltage VDET supplied to a non-reverse input terminal with the voltage VREF of a reverse input terminal. The VDET increases in response to the wear of the magnetic head. When the VDET exceeds the VREF, the output of the amplifier 15 is set at an H level.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a head wear detection circuit for detecting wear of a rotary magnetic head driven by a current, and particularly to a head wear detection circuit for detecting wear of a rotating magnetic head driven by a constant current. The present invention relates to a head wear detection circuit for detecting head wear in a magnetic recording device equipped with a video rotary head driven at a constant voltage.

[Conventional technology]

In recent years, video tape recorders (so-called VTRs) equipped with a rotating magnetic head for video signals and a rotating magnetic head for audio signals have been supplied on the market.

In such a VTR, the audio signal is FM modulated and recorded by a rotating magnetic head driven by a constant current so as to reach the deep layer of the magnetic layer of the magnetic tape.
In the video signal, the chroma signal is low-frequency converted and the luminance signal is r
4 modulation and is superimposed on the surface of the magnetic tape on which the audio signal is recorded by a rotating magnetic head driven at a constant voltage. A method of recording, for example, video signals and audio signals independently in the surface layer and the deep layer of the magnetic layer of a magnetic tape in this way is generally called a deep recording method.

By the way, as each rotary magnetic/magnetic head of this deep recording type VTR wears out due to aging, the characteristics of the head change and the inductance generally decreases. For this reason,
The recording current of the video head driven at a constant voltage increases, and the magnetic flux spreads as the head shape changes. If the video recording current becomes excessive in this way, not only will image deterioration such as increased crosstalk and overmodulation occur, but the video signal will also be magnetically recorded deep into the magnetic layer of the tape. As a result, the recording area of the audio signal is eroded, the output level of the reproduced audio signal is lowered, and the S/N ratio is degraded. The deterioration in sound quality of this reproduced audio signal is more remarkable than the deterioration in image quality of the video signal, and in particular, the adverse effects such as so-called sound breakage due to FM modulation are significant.

To prevent this, it is necessary to detect changes in head inductance and correct the recording current of the video head.
It is extremely difficult to directly detect changes in inductance of the video head driven at a constant voltage.

Therefore, the applicant first filed the patent application No. 58-188856.
No. 2, proposes a recording apparatus that detects the impedance of an audio head that is in the same state of wear as the video head, estimates the wear state of the video head, and corrects the recording current for the video head.

In this first technique, the terminal voltage of the audio head is a reference voltage obtained by dividing the power supply voltage, and the change in the head terminal voltage is extremely small.

Since a detection circuit capable of detecting such minute voltage changes is extremely sensitive, it is easily affected by the content of the recorded audio signal, changes in temperature, fluctuations in power supply voltage, etc.

[Problem that the invention seeks to solve]

In this way, for example, in a conventional deep recording VTR,
Wear of the video head lowers the head impedance, and the recording current becomes excessive, causing various adverse effects.

In order to prevent this, the terminal voltage of the audio head is compared with a fixed reference voltage (reference voltage) to detect a drop in the impedance of the audio head, and from this detection result, the state of wear of the video head is estimated, and the wear condition of the video head is estimated. It has previously been proposed to correct the recording current for . However, in such prior art, since the reference voltage is created by dividing the power supply voltage, it is easily subject to fluctuations due to external factors such as other circuits and the power supply, causing malfunctions.

In view of these points, the present invention provides a head wear detection system that uses a simple circuit configuration to prevent the occurrence of malfunctions without being affected by fluctuations due to the above-mentioned external factors, and to perform reliable head wear detection. The purpose is to provide circuits.

[Means for solving the problem] That is, the characteristics of the head wear detection circuit according to the present invention are as follows.
means for detecting a terminal voltage of a rotating magnetic head driven with a constant current; means for outputting a reference voltage controlled in accordance with the current supplied to the rotating magnetic head; and a comparison means for comparing the voltage from the grass voltage output means, and the amount of wear of the rotary magnetic head is detected based on the output from the comparison means.

In this case, by incorporating a rotary magnetic head driven by a constant current and a rotary magnetic head driven by a constant voltage into the same rotary head device, the amount of wear of the constant voltage drive head can be calculated from the amount of wear of the constant current drive head. It can be estimated 1.
It goes without saying that the recording current of the constant voltage drive head can be controlled to an appropriate value depending on the amount of wear.

[For production]

According to the head wear detection circuit having the above-mentioned configuration, the reference voltage can vary depending on the current supplied to the rotating magnetic head driven at a constant current.

Even if the head terminal voltage detection output fluctuates due to power fluctuations or other external factors, the above reference voltage will also fluctuate to follow the fluctuations in the detection output, ensuring stable head wear detection that is always reliable and free of malfunctions. .

[Embodiment] FIG. 1 is a block circuit diagram showing the basic configuration of a head wear detection circuit and its peripheral circuits, which is a first embodiment of the present invention.

In FIG. 1, an audio signal from an audio signal input terminal 1 is sent to an FM modulator 2 and subjected to FM modulation, and then amplified by a recording amplifier 3 and connected to a power supply terminal 6, and the collector is sent to an audio head 7. It is connected to the primary coil of a signal transmission transformer, etc.

Next, a head wear detection circuit 10, which is a basic first embodiment of the present invention, includes a voltage detection circuit 11 connected to the collector of the transistor 4, and a voltage detection circuit 11 connected to the emitter of the transistor 4 via an amplifier 12. a voltage detection circuit 13;
An operational amplifier (so-called operational amplifier) 1 for comparing the detected voltages from these voltage detection circuits 11 and 13 with each other.
5. Note that a variable resistor 14 for gain adjustment is connected to the amplifier 12.

The output terminal of the operational amplifier 15 of this head wear detection circuit 10 is connected to the switching transistor 2 via a resistor 21.
The emitter of this transistor 22 is grounded via a resistor 23, and the collector is connected via a resistor 24 to a signal transmission line (not shown) of a video signal recording system.

In the circuit having the above configuration, the transistor 4 has a constant current on the emitter side of the rotating magnetic resistor 4 via a rotary transformer, etc., so the voltage across the emitter resistor 5 hardly changes, and this terminal Amplify voltage 1
2, the voltage detection circuit 13 detects the DC reference voltage V.
Obtained REF. That is, the amplifier 12 and the voltage detection circuit 13 are used as means for obtaining a reference voltage.

In the operational amplifier 15, the detection voltage VDET supplied to the non-inverting input terminal is compared with the reference voltage VREF of the inverting input terminal, and as the detection voltage VDET increases with the head wear mentioned above, the reference voltage increases. When VREF is exceeded, the output of the operational amplifier 15 becomes H'' (high level), and the switching transistor 22 is turned on to shunt the recording current of the video signal system, for example.
The recording current supplied to the video head is attenuated.

In this case, since the audio head and video head installed in the same rotating head device wear out to the same degree, the increase in video recording current when the inductance of the video head driven by a constant voltage decreases due to head wear can be avoided. This can be prevented by turning on the transistor 22, and correction is performed so that the recording current is always close to the optimum value, thereby preventing adverse effects such as deterioration of the SN ratio of the audio signal and sound breakage.

By the way, since the voltage detection circuit 11 is extremely sensitive and can detect minute voltage changes due to head wear, it is not affected by the contents of the recorded audio signal, changes in temperature, fluctuations in power supply voltage, etc. However, if a constant voltage obtained from, for example, a resistive voltage divider circuit is used as the reference voltage, malfunction may occur due to voltage fluctuations caused by the external factors. Therefore, in the present invention, the terminal voltage of the resistor 5 on the emitter side of the transistor 4 that drives the audio head 7 with a constant current is detected by the voltage detection circuit 13 and set as the reference voltage VREF. Since it is controlled according to the current supplied to the head terminal 7, it fluctuates in accordance with fluctuations in the head terminal voltage detection output caused by the above-mentioned external factors, and stable operation is possible.

Next, FIG. 2 shows, as a second embodiment of the present invention, an example of a head wear detection circuit configured to use two different reference voltages and to switch the recording current of a video head signal, for example, in two stages. ing.

In FIG. 2, a transistor 4 provided upstream of the head wear detection circuit 30 corresponds to the transistor 4 in FIG. 1 described above, and is for driving the audio head 7 with a constant current. A voltage detection circuit 31 for detecting head terminal voltage is connected to the collector of this transistor 4.
A voltage detection circuit 33 for obtaining a reference voltage is connected to the emitter via an amplifier 32. A variable resistor 34 for gain control is connected to the amplifier 32. next.

It is sent to each non-inverting input terminal of two operational amplifiers 35 and 36. The output from the voltage detection circuit 33 is sent to the inverting input terminal of the other operational amplifier 36 as a second reference voltage VREF2.

Each operational amplifier 35 of such head wear detection circuit 30
．． Various configurations can be considered in order to switch the video head recording current into two stages based on the output from the switching transistor 36. For example, in FIG. 2, two switching transistors 4
1.42 is controlled to be turned ON and OFF by the outputs from the operational amplifiers 35 and 36, respectively. That is, the output terminal of the operational amplifier 35 is connected to the base of a transistor 41 via a resistor 43, the emitter of this transistor 41 is grounded via a resistor 44, and the collector is connected to a terminal 46 via a resistor 45.

Further, the output terminal of the operational amplifier 36 is connected to the base of a transistor 42 via a resistor 47, the emitter of this transistor 42 is grounded via a resistor 48, and the collector is connected to the terminal 46 via a resistor 49. By connecting this terminal 46 to a video signal transmission line, for example, a video recording current can be shunted depending on whether the transistors 41 and 42 are turned on or off, and the video signal level can be switched and controlled.

Next, FIG. 3 shows a specific circuit configuration example of the head wear detection circuit 30 shown in FIG. 2, and parts corresponding to those in FIG. 2 are given the same reference numerals.

3, an input terminal 51 is connected to the collector of the transistor 4 shown in FIG. 2, an input terminal 52 is connected to the emitter of the transistor 4, and a voltage detection circuit 31 is connected to the collector of the transistor 4. The AC voltage is taken out through the capacitor 53 and connected to the diode 5.
4 and rectified by a transistor 55, the output from the transistor 55 is passed through a low-pass filter 56 consisting of a capacitor and a resistor, thereby outputting a detection voltage VDET corresponding to the average value of the waveform. There is. Since the collector side of the transistor 4 is an inductance load caused by the head 7, the FM modulation signal when an audio signal is input is subjected to AM modulation according to the frequency characteristics of the inductance. By detecting the average value of the waveform, even if the waveform is subjected to AM modulation and its maximum amplitude changes, the detected voltage VDET and It becomes possible to maintain good followability with the reference voltage (VREF2, etc.).

The configuration of the voltage detection circuit 31 described above is the same for the voltage detection circuit 33.

By the way, even if the voltage detection circuits 31 and 33 having such a configuration are used, in cases where the average value is not necessarily constant, such as in an actual audio signal, or where the waveform is distorted during a large input, etc., the above standards cannot be met. It is difficult to cause the voltage to completely follow changes in the detection voltage VDET. This is caused by the difference in frequency characteristics between the collector side and the emitter side of the transistor 4, so as a countermeasure, in this embodiment, the coil 63 is connected in parallel with the collector resistor 62 of the transistor 61 constituting the amplifier 32. It is set up. By appropriately selecting the inductance value of the coil 63 and the resistance values on the collector and emitter sides, the frequency characteristics of the amplifier 32 can be made to closely approximate the frequency characteristics of the head 7, and the detected voltage fluctuation during audio input can be Reference voltage VREF
I and VREF2 are offset by faithfully following the detection voltage VDET.

Next, in order to improve fluctuations due to temperature changes, the amplifier 3
A thermistor circuit 65 using a thermistor for temperature compensation is provided in parallel with the collector resistor 62 of No. 2. This thermistor circuit 65 can compensate for input level fluctuations due to the temperature characteristics of the impedance of the head 7, and also
Compensation for the temperature characteristics of 2 itself is also performed.

In addition, in the specific circuit configuration shown in FIG. 3, the adjustment range by the variable resistor 34 on the emitter side of the amplifier 32 is optimized, the hysteresis added to the operational amplifiers 35 and 36 is optimized, and further, A time constant circuit is provided to prevent malfunctions due to level changes when the VTR starts recording.

The overall operation of the second embodiment of the present invention having the above configuration will be explained with reference to FIGS. 4 and 5.

First, Fig. 4 shows the relative output level of the reproduced audio signal when the audio signal and video signal are recorded overlappingly using the above-mentioned rotating head device, and the vertical axis shows the relative output level in dB (decibels), and the horizontal axis shows the relative output level of the reproduced audio signal in dB (decibels). indicates the amount of time for wrapping using a so-called wrapping tape in minutes as an amount corresponding to the amount of head wear. here,
It is assumed that the total life of the head in normal usage corresponds to, for example, about 40 minutes of the above-mentioned lapping process.

In FIG. 4, curve A represents the state in which the video signal recording current is not switched and controlled, and curve B represents the state in which the video signal recording current is controlled to be switched to the first shade (for example, only the transistor 41 is controlled to be ON). B controls the switching of the video signal recording current to the second stage (for example, the transistor 42 is also
(N controlled) states are shown respectively.

Next, FIG. 5 shows the detected voltage Vf)ET when the head inductance changes due to wear of the audio head.
The amount of change in the initial detection voltage setting value (setting voltage) VSET
The horizontal axis shows the lapping time (in minutes) corresponding to the amount of head wear. The above set voltage VSET in this case
corresponds to the reference voltage VREF mentioned above,
For example, set the first reference voltage Vnyx to a potential of 250 mV from the set voltage VSET.
The second reference voltage VREF2 is distributed to the potential of 0077LV.

In these figures 4 and 5, since head wear progresses at the same rate for both the video head and the audio head, the video signal recording current increases due to video head wear, and the audio signal recording area in the deep part of the tape is eroded. As a result, the relative output level of the reproduced audio signal decreases as shown in FIG. 4, and the VSET-VDET difference voltage decreases as shown in the audio signal.

In this way, head wear progresses, and at a time point Tt when the relative output level of the reproduced audio signal decreases by, for example, 4 dB from the initial value, the detection voltage VilET changes to the first level.
Assuming that the reference voltage VREFI is reached, after this time point Tl, the output from the operational amplifier 35 serving as a comparison means becomes "H" (/1 level), the switching transistor 41 is turned on, and the video signal system, for example, is turned on. As the recording current is shunted through the transistor 41 and the recording level of the video signal is lowered, the relative output level of the reproduced audio signal increases by, for example, 4 dB and returns to its initial value, as shown in FIG.

After the switching operation of the video signal recording current in the first stage as described above is performed (after time Tl), the head wear progresses further, and the relative output level of the reproduced audio signal decreases again by, for example, 4 DB from the initial value. At time T2, the detection voltage VDET becomes the second reference voltage VREF2.
Assuming that after this time T2, the output from the operational amplifier 36 as a comparison means also becomes H" (high level), and the switching transistor 42 is turned on.
Operate. Therefore, for example, the recording current of the video signal system is shunted through the transistors 41 and 42, and the recording level of the video signal decreases, so that the relative output level of the reproduced audio signal increases by, for example, 4 dB as shown in FIG. and returns to the above initial value again.

In this way, a two-step switching control operation of the video signal recording current is performed, but even if the detection voltage vnEτ fluctuates due to fluctuations in the power supply voltage or other external factors, the reference voltage will follow this. Since VREFI and VREF2 fluctuate, the voltage difference between them is kept stable and no malfunction occurs.

In the above operation description, each reference voltage vREFl +V
The specific value of REF2 is predetermined in consideration of the relative output level of the reproduced audio signal, and in this embodiment, it is set to the detection voltage VDET when the relative output level decreases by 4 dB from the initial value. Based on VREFI
, VREF2 is determined, but it goes without saying that the amount of decrease in relative output level and each reference voltage in this case may be set to appropriate values in consideration of actual head wear, head life, etc.

It should be noted that the present invention is not limited to the above-mentioned embodiments. For example, the video recording current may be controlled in three or more stages by comparing the detected voltage with three or more reference voltages. Further, the video recording current may be continuously controlled in accordance with a differential voltage output obtained by comparing the detected voltage and a reference voltage.

〔Effect of the invention〕

According to the head wear detection circuit according to the present invention, the reference voltage is determined according to the current supplied to a constant current driven head such as a rotary audio head, and varies in accordance with fluctuations in the detected voltage due to external factors. Therefore, the voltage difference between the detection voltage and the reference voltage can be obtained as a voltage that corresponds only to the amount of head wear that is not affected by the above external factors, and it is possible to obtain a voltage difference between the detection voltage and the reference voltage that corresponds only to the amount of head wear that is not affected by the above external factors. Even if a sensitive voltage detection circuit is used, stable and reliable operation is possible.

In addition, when an audio head driven by a constant current and a video head driven by a constant voltage are built into the same rotating head device, such as in a so-called deep recording type VTR, the video head By estimating the amount of wear on the head and controlling the recording current to the video head in accordance with this amount of wear, optimal recording and reproduction of both video and audio signals can be performed, and various adverse effects caused by signal deterioration can be effectively prevented.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing a first embodiment of the present invention;
FIG. 2 is a block circuit diagram showing a second embodiment of the present invention;
FIG. 3 is a circuit diagram showing a specific circuit configuration example of the second embodiment, and FIGS. 4 and 5 are graphs for explaining the operation of the second embodiment.

Claims (1)

[Claims] means for detecting terminal voltage of a rotating magnetic head driven by a constant current; means for outputting a reference voltage controlled in accordance with the current supplied to the rotating magnetic head; and voltage from the voltage detecting means and the reference. A head wear detection circuit comprising: comparison means for comparing the voltage from the voltage output means, and detects the amount of wear of the rotating magnetic head based on the output from the comparison means.